1. Field of the Invention
This invention pertains to gyroscopes. More particularly it pertains to gyroscopes used in inertial measuring units (IMU) for inertial navigation and to such inertial measuring units.
2. Description of Prior Art
Inertial navigation and guidance systems are used to measure changes in position and velocity of vehicles such as aircraft, missiles and ships. These changes in position and velocity are used by the inertial systems and/or associated equipment to generate current position and velocity information and in some applications to generate steering and propulsion control signals in accordance with predetermined guidance instructions.
Errors are always present in the measurement of position and velocity changes by such inertial systems. A significant part of the error in such measurements by inertial measuring units (IMU) is due to imperfections in the performance of the gyroscopes, the accelerometers and the associated equipment which form a part of the IMU.
An operating technique often used in the prior art to reduce the effect of certain types of errors in IMU's is to cause the stable platform which is a part of the IMU to rotate about its azimuthal axis (the azimuthal axis is defined here as being parallel to the direction of apparent acceleration). See for instance, U.S. Pat. No. 3,214,981. Azimuthal rotation, or carrousel operation, converts the effect of level gyro bias, accelerometer bias and accelerometer axis misalignment from that of a fixed error to that of an error which varies sinusoidally at the platform rotation frequency. Since, in the self-alignment mode, azimuthal alignment (coincidence of computer axes and platform axes about the vertical) accuracy is limited by level gyro bias uncertainty, carrousel operation leads to improved azimuthal alignment.
The amount of IMU accuracy improvement resulting from carrousel operation depends upon the IMU application e.g. ballistic missile guidance, aircraft or ship navigation. The amount of improvement depends upon the error correlation time of the gyro bias error and on other errors which are reduced by azimuthal rotation, and upon the azimuthal rotation rate.
In the prior art, stable platform azimuthal rotation normally is caused by electromagnetic torquers operating on the stabilization gyros. Electromagnetic torquers are limited in range and accuracy so that in practice the azimuthal rotation rates obtained in the prior art in precise applications are limited to 15 to 90 degree per hour. Higher IMU accuracy, however, would be achieved if precise means were available for causing a high rate of rotation of the stable platform. For example, if a precise rotation rate of 720.degree./hr were used, then bias stabilities of only 30 minutes rather than 4 to 24 hours would be needed to obtain improved accuracy of operation.